why we use postgresql in openerp? What are the benefits of using postgresql?
It's because the size of the application. If you want to use large databases you have to use it. It supports rollbacks something that It's really useful with OpenERP. And the referential integrity, mysql has it with InnoDB but it's really problematic when you want to export it.
There could be another 2 replies after this disagreeing with me. However, PostgreSQL is seen to be better when utilising "enterprise" applications. If you just want a quick dirty basic database driven website, MySQL would be much better.
Personally, if I want a full enterprise DBMS, I go PostgreSQL all the way. But if I want a small/basic application or website with a database backend, then I'd go MySQL. This is not based on volume of users or visitors, but mainly what you want from a DBMS. Or if I have a big website that's not needing extra DB features PostgreSQL need and just need a DB storage engine, then MySQL is the way to go.
I am sure some could argue MySQL could easily be used for OpenERP I'm sure.
Additionally, there is the whole Oracle owning MySQL issue, but that's less of an issue now with Maria DB (fork of MySQL).
I have added a link, again some are personal views. But it gives you an idea: